Rubbing seal for high temperature ceramics

ABSTRACT

A seal member designed to rub against a rotating ceramic regenerator is made by applying a surface layer consisting essentially of a glazing material such as a fluoride of lithium, sodium, potassium or calcium and a matrix material such as zinc oxide to a metal substrate. A bonding layer of nickel-aluminide can be applied to the substrate and an intermediate layer of nickel oxide and calcium fluoride can be included between the nickel-aluminide layer and the surface layer. The glazing material forms a low friction glaze at a temperature depending primarily on the composition thereof with lithium fluoride compositions producing useful glazes between about 450*-900* F., calcium fluoride compositions producing useful glazes at about 700*-1,800* F., and sodium and potassium compositions producing useful glazes in intermediate temperature ranges.

United States Patent [191 Baoet al.

[451 July 17, 1973 RUBBING SEAL FOR HIGH TEMPERATURE CERAMICS [21] App]. No.: 854,397

[52] US. Cl. 277/235 R, 106/39 R, 117/129,

277/96 [51] Int. Cl Fl6j 15/16 [58] Field of Search 117/105, 129, 70 C [56] References Cited UNITED STATES PATENTS 3,481,715 12/1969 Whalen et a1 117/1052 Primary Examiner-Laverne D. Geiger Assistant Examiner-Robert 1. Smith Attorney-John R. Faulkner and Glenn S. Arendsen 57 ABSTRACT A seal member designed to rub against a rotating ceramic regenerator is made by applying a surface layer consisting essentially ofa glazing material such as a fluoride of lithium, sodium, potassium or calcium and a matrix material such as zinc oxide to a metal substrate. A bonding layer of nickel-aluminide can be applied to the substrate and an intermediate layer of nickel oxide and calcium fluoride can be included between the nickel-aluminide layer and the surface layer. The glazing material forms a low friction glaze at a temperature depending primarily on the composition thereof with lithium fluoride compositions producing useful glazes between about 450900 F., calcium fluoride compositions producing useful glazes at about 7001,800 F1, and sodium and potassium compositions producing useful glazes in intermediate temperature ranges.

8 Claims, 3 Drawing Figures PATENTEDJULI H915 3.746.352

F'IG.2

28-REGENERATOR com:

26" SURFACE LAYER 24-INTERMEDIATE LAYER ZO-METAL SUBSTRATE 22 BONDING LAYER FIG.3

. E] -28-REGENERATOR cons aiming} 2s SURFACE LAYER 4 V x V 32- BONDING LAYER -20-METAL SUBSTRATE RUBBING SEAL FOR HIGH TEMPERATURE CERAMICS BACKGROUND OF THE INVENTION This invention relates to scaling members of the type disclosed in US. Pat. application, Ser. No. 613,920,

filed Feb. 2, 1967, now US. Pat. No.c3,48l,7l5 and assigned ,to the assignee of this application.

Rotary regenerators for gas turbine engines are being made of a ceramic material capable of effective heat transfer at elevated temperatures. Typical ceramic materials useful in such regenerator-s include .petalite, glass-ceramics, .spodumene or other refractory materiials having suitable hightemperature properties. Cercor made of metal oxides or carbides are capable of withstanding the expected temperatures; in general, however, these materialsresulted in relatively high wear of the ceramic core.

SUMMARY OF THE INVENTION This inventionrprovidesa seal havinggood oxidation resistance and a low coefficient of friction and low wear when rubbing against a ceramic regenerator. The

seal comprises a metal substrate having good oxidation resistanceat the anticipated temperatures and-acoefflcient of thermal expansionmatched as closelyas possible to the thermal expansion properties of the coatings applied thereto asdescribedbelow.*Atypicalsteel substrate is made from nickel chromium stainless steels such-as 430SS or high temperature alloys such as Incoloy600or 750 sold by lnternationalNickel Co. or Hastalloy X sold by Union Carbide Co. A surface layer consisting essentially of a; glazing material and a'matrix material non-abradable to theceramic is applied to one side of the'substrate where .thesurface layer will contact the ceramic regenerator. Glazing'materials consist essentially of lithiumrfluoride, sodium fluoride, potassium fluoride, or calciumfluoride.0ther fluorides or chlorides can be added to theglazingmaterials to reduce the glazing temperature and thereby lower the operating range thereof.

An intermediate layer of nickel oxide and calcium fluoride .can .be; placed in contact with the surface layer lithium fluoride. produceaa lowfriction, low wear glaze i at temperatures between about 500 F. and 900 "F.

Similarly, sodium and potassium fluoride materialsproduceusefull glazes at temperatures between about 650 l,l00 F., and calcium fluoride materials produce usefulaglazesattemperatures between about-800 l;800:F. Addingup toabout-weight percent of other fluorides or chlorides of. lithium, sodium,:potassium and calcium to the'glazing materials reduces both the minimum and maximum temperatures by as much as 500 F. Such additions. preferablyare fluorides since the fluorides produce glazes having better overall properties.

Matrix materials useful in the surface layers include zinc-oxide, cuprous oxide and stannous oxide for the lithium, sodium, and potassium fluoride glazing materials. A higher temperature glazing materal also can be used as a matrix material; for example, fluorides of sodium, potassium, and calcium can be used as matrix materials forlithium fluoride glazing materials, fluorides of potassium andcalcium can be used as matrix materials for sodium fluoride, and calcium fluoride matrix material can beused with potassium fluoride glazing materials. Mixtures of these fluorides and oxides compatible with the ceramic of the regenerator also can beused asmatrix materials.

Zinc oxide, cuprous oxide, stannous oxide, nickel oxide, strontium zirconate, barium zirconate or barium titanate'areusedas matrix materials for the high temperature calcium fluoride glazing materials. Brass or bronze powders can be used to supply matrix materials containing both cuprous oxide and zinc oxide or cu- :prous oxide and stannous oxide. Small amounts of carbon preferably are added to matrixmaterials containing cuprous oxide to prevent complete conversion of cuprous oxide to cupric oxide which increases friction considerably. The carbon generally is added as a metallic carbon composite suchas a nickel carbon composite obtainable from Sherritt Gordon'Co. The nickel carbon composite containsabout25 weight percent carbonland is used in proportions ranging up to about20 percent of the surface layer.

AboutS-QO weight'percent of glazing material with the balance matrix material produces surface layers having good friction, low wear,and, good adhesion to the bonding or intermediate layers. The minimum temperature -at which theglazing material produces the glazecan be lowered by usinge u'tectic mixtures of the glazing material and the :matrix material.

Each of the layers can be applied by a plasma spraying'technique. The bonding layer is prepared by mixing .powders of the ingredients and spraying the'powder ontoa fully annealed or aged substrate. An intermediate layer is appliedby ball milling nickel oxide and calslightly oxidizing. atmosphere made up primarily of nitrogen, argomxenon, or helium that prevents any reduction of nickel oxide, dry grinding the sinteredproduct, and spraying the resulting powder on the bonding layer. An inert atmosphere prevents any formation of calcium oxide and is preferred. Plasma spraying with an inert gas also is preferreduSurface layers are prepared in much the samemanner from powders of the appropriate components. Intermediate and surface layers also can beyprepared by mixing powders with a suitable binder such as-gum arabic orpolyvinyl'alcohol in a waterbase slurryand agglomerating the mixture in a spray drier. The resultingmixture is applied to a substrate and heatedtoburn off the binder.

At the operating temperatures and pressures encountered inthe regenerator environment of agas turbine engine, thelglazing materials develop a glaze surface on theseal member that has satisfactory friction and wear when'rubbing against a ceramic regeneratorJFriction coefficients below about 0.45 are considered to be satisfactory for use in the gas turbine engine. Small amounts of the glazing materials can transfer onto the rubbing surface of the regenerator to produce a similar glaze there which reduces wear rates. Materials such as boric oxide that are detrimental to the ceramic regenerator core are avoided in any of the layers of the seal. The best combination of friction, wear and seal integrity is provided by surface layers comprising lithium fluoride glazing material and zinc oxide matrix material for temperatures between about 450 900 F. and by calcium fluoride zinc oxide surface layers for higher temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a regenerator seal of this invention showing the two semi-circular peripheral seal members and the diagonal cross arm seal member.

FIG. 2 is a cross-section of a four component seal member having a nickel-aluminide bonding layer, a nickel oxide-calcium fluoride intermediate layer and a surface layer.

FIG. 3 is a cross-section of a three component seal member having a nickel chromium bonding layer and a surface layer.

DETAILED DESCRIPTION Referring to the drawings, a seal assembly for a disc type rotating regenerator of a gas turbine engine comprises two semi-circular rim seals 10 and 12 separated by a straight cross arm seal 14. The seal assembly is mounted in the housing of a gas turbine engine and a disc type regenerator is mounted for rotation on top of the seal assembly with the lower surface of the regenerator sliding on the seal assembly. During engine operation, relatively cool gases from the engine compressor flow downward through the sector of the regenerator above passage 16 and into passage 16 which conducts the gases to the engine combustion chambers. Hot combustion gases flow upward through passage 18, through the sector of the regenerator above passage 18, and out the engine exhaust port (not shown). Rotation of the regenerator transfers heat from the hot combustion gases leaving passage 18 to the relatively cool gases entering passage 16.

As shown in FIG. 2, any of seal members 10, 12 or 14 comprise a substrate 20 having a bonding layer of nickel-aluminide 22 on one surface. An intermediate layer 24 consisting essentially of nickel oxide and calcium fluoride is located on top of bonding layer 22. Intermediate layer 24 preferably is about 0.003 inch 0.010 inch thick and preferably consists of about 55-95 weight percent nickel oxide with the remainder calcium fluoride.

A surface layer 26 is located on top of intermediate layer 24. Good results are obtained with a surface layer about 0.005 0.050 inch thick and containing about l-90 weight percent glazing material with the balance matrix material. Best friction and wear properties are obtained with about l0-40 weight percent glazing material. In the alternate construction shown in FIG. 3, a bonding layer 32 consisting essentially of a nickelchromium alloy is applied to one surface of substrate 20 and surface layer 26 then is applied directly to bonding layer 32. Cross arm seal member 14 generally operates at a higher temperature than peripheral seals and 12, and surface layers for the cross arm seal use calcium fluoride glazing materials. Lithium, sodium, or

potassium fluoride glazing'materials or glazing materials containing calcium fluoride plus one of the other fluorides usually are used in the surface layers of the peripheral seals.

' EXAMPLE I A bonding layer of nickel-aluminide is applied to a 43088 stainless steel substrate by mixing powders of nickel and chromium together in a ball mill and flame spraying the mixture onto the substrate. 60 weight parts of copper powder are ball milled with 40 weight parts of lithium fluoride powder to obtain a mixture having a particle size of-l to +325 mesh. This mixture is flame sprayed onto the bonding layer in an inert atmosphere to obtain a surface layer having a thickness of about 0.025 0.030 inch. The seal member then is placed in an oxidizing atmosphere at about 500 650 F. for about 2 hours to convert the copper powder to cuprous oxide matrix material and the surface layer is ground to approximately a 170 rms finish.

The resulting seal member is installed in a test rig in which a ceramic regenerator is loaded onto the surface layer of the seal with a load of about 7 psi. At 500 F. the friction coefficient of the regenerator-seal arrangement averaged about 0.12 0.15 and the wear rate of the surfaces averaged about 0.0007 inch per I00 hours. At 600 F. friction coefficients were 0.l2 0.18 and wear rates were 0.0005 0.001 inch per 100 hours. At 750 F. friction coefficients were 0.2 0.4 and wear rates were 0.0015 inch per 100 hours. Analysis revealed that the increased friction and wear at 750 F. resulted from conversion of most of the cuprous oxide to cupric oxide. The seal member was considered satisfactory for use at temperatures below about 700 F.

EXAMPLE II A surface layer of weight parts copper and 20 weight parts lithium fluoride was prepared in the manner of Example I. At 600 F. in the test rig, the seal member has a coefficient of friction of 0.15 and a wear rate of 0.0005 inch per 100 hours.

EXAMPLE III About 10 weight parts of a nickel carbon composite (75 weight percent nickel and 25 weight percent carbon) were added to weight parts of the powder used to make the surface layer of Example I, and a seal member was made with a surface layer of theresulting mixture. At 750' F. in the test rig, the seal member had a friction coefficient of 0.2 and a wear rate of 0.0008 per hours. Analysis showed that the reduced friction and wear rate at the test temperature was caused by the inhibiting effect of the carbon on the formation of cupric oxide.

EXAMPLE IV A 43OSS steel substrate was flame sprayed with a powder of nickle and aluminide to produce a bonding layer of nickel-aluminide. 80 weight parts of high purity nickel oxide powder was mixed with 20 weight parts of calcium fluoride powder in a ball mill for about 4-12 hours. Both powders initially had a particle size of to +325 mesh. After milling, the mixture was sintered in an inert atmosphere at 2,5003,000 F. and the sintered product was ground to produce a powder having a particle size of-l00 to +400 mesh. Sintering was carried out carefully to prevent reduction of the nickel oxide and formation of calcium oxide. The resulting mixture was flame sprayed onto the bonding layer to produce an intermediate layer.

A'surface layer was produced by mixing in a ball mill the powder used to produce the intermediate layer with about weight percent of dry potassium chloride powder having a corresponding particle size. The resulting mixture is flame sprayed onto intermediate layer 24 to produce a surface layer containing 72 weight percent nickel oxide, 18 weight percent calcium fluoride and 10 weight percent potassium chloride and having a thickness of about 0.020 inch. A surface finish of about 40 rms was produced on the surface layer by dry grinding.

At 500-700 F. in the test rig, this seal generated a friction coefficientof about 0.4 0.25 and displayed a wear rate of 0.001 inch per 100 hours. The friction coefficient dropped to about 0.12 at 800 F. and to about 0.10 at 900 F. while the wear rate dropped slightly. For purposes of comparison a surface layer prepared according to this example but lacking potassium chloride exhibited friction coefficients in excess of 0.45 at temperatures below 800 F. Thus the addition of potassium chloride reduced the lower threshold of the useful temperature range by about 500 F.

EXAMPLE V A surface layer of 80 weight percent strontium zirconate as the matrix material and 20 weight percent calcium fluoride as the glazing material was substituted for the surface layer of Example IV. Friction coefficients ranged from 0.4 to 0.35 and wear rates averaged about 0.001 inch per 100 hours at temperatures between700 and 1,400 F. Similar results were obtained at l,400 F. when barium titanate was substituted for the strontium zirconate. Substituting barium zirconate for the strontium zirconate produced approximately the same coefficient of friction and a wear rate of about 0.003 inch per 100 hours at l,400 F.

EXAMPLE VI A surface layer of 80 weight percent zinc oxide as the matrix material and 20 weight percent calcium fluoride as the glazing materialwas substituted for the surface layer of Example IV. At 1,400F. the friction coefficient was about 0.3 and the wear rate was about 0.003 inch per 100 hours. 1

Thus this invention provides seals for use againsta ceramic regencrator that have both satisfactory friction and long life at the temperatures and in the highly corrosive gases encountered in the engine. The seals have a low wear rate when rubbing against the ceramic regenerator and maintain seal integrity despite the widely varying temperatures encountered in the engine.

What is claimed is:

l. Aseal member having a low coefficient of friction and low wear when rubbing against a ceramic material at temperatures above about 500 F. comprising a metal substrate having a surface layer attached to one surface for rubbing against said ceramic material, said surface layer consisting essentially of a glaze producing material selected from the group consisting of lithium fluoride, sodium fluoride, and potassium fluoride and a non-abradable matrix material selected from the group consisting of zinc oxide, cuprous oxide, stannous oxide, calcium fluoride, potassium fluoride and sodium fluoride, any of said fluorides in. said matrix material having a glazing temperature higher than the glazing temperature of said glaze producing material.

2. The seal member of claim 1 in which the glaze producing material consists essentially of lithium fluoride.

3. The seal member of claim 2 in which the matrix material consists essentially of zinc oxide 5 4. The seal member of claim 31in which the surface layer contains about 10-40 weight percent glazing material and the balance matrix material.

5. The seal member of claim 1. in which the surface 1 layer contains about 10-40 weight percent glazing ma terial and the balance matrix material.

6. A seal member having a low coefficient of friction and low wear when rubbing against a ceramic material at temperatures above 700F. comprising a metal substrate having a surface layer attached to one surface, said surface layer consisting essentially of a glaze producing material containing calcium fluoride and a nonabrading matrix material selected from the group consisting of zinc oxide, cuprous oxide, barium zirconate, strontium zirconate, and barium titanate, said surface layer being free of calcium oxide.

7. The seal member of claim 6 in which'the matrix material is zinc oxide.

8. The seal member of claim 6 in which the surface layer contains about 10-40 weight percent of glaze producing material with the balance: matrix material.

UNITED PATENT OFFICE CERTIFKCATE (W CORRECTION Patent No. 33 7M6 9 352 Dated y 7 a 973 Inventor(s) v. D. Rao et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title Page, Column l, line 3, cancel Vemulapa lli DurganageswaruBao" and substitute -vemulapalli Durganageswar' Rao---.

Signed and sealed this 1st day of October 197 (SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner 'of Patents USCOMM-DC 60376-P69 U.S, GOVERNMENT FRINTING (OFFICE: 1969 0-366-3.

FORM PC4050 (10-69) 

2. The seal member of claim 1 in which the glaze producing material consists essentially of lithium fluoride.
 3. The seal member of claim 2 in which the matrix material consists essentially of zinc oxide.
 4. The seal member of claim 3 in which the surface layer contains about 10-40 weight percent glazing material and the balance matrix material.
 5. The seal member of claim 1 in which the surface layer contains about 10-40 weight percent glazing material and the balance matrix material.
 6. A seal member having a low coefficient of friction and low wear when rubbing against a ceramic material at temperatures above 700*F. comprising a metal substrate having a surface layer attached to one surface, said surface layer consisting essentially of a glaze producing material containing calcium fluoride and a non-abrading matrix material selected from the group consisting of zinc oxide, cuprous oxide, barium zirconate, strontium zirconate, and barium titanate, said surface layer being free of calcium oxide.
 7. The seaL member of claim 6 in which the matrix material is zinc oxide.
 8. The seal member of claim 6 in which the surface layer contains about 10-40 weight percent of glaze producing material with the balance matrix material. 